In acoustic well logging, it is customary to measure the compressional wave velocity of earth formations surrounding boreholes. A conventional compressional wave velocity logging system includes a cylindrical logging sonde for suspension in a borehole liquid, a source connected to the sonde for generating compressional waves in the borehole liquid, and one or more detectors connected to the sonde and spaced apart from the compressional wave source for detecting compressional waves in the borehole liquid. A compressional wave in the borehole liquid generated by the source is refracted into the earth formation surrounding the borehole. It propagates through a portion of the formation and is refracted back into the borehole liquid at a point adjacent to the detector and is then detected by the detector. The ratio of the distance between the source and detector to the time between generation and detection of the compressional wave yields the compressional wave velocity of the formation. The distance between source and detector is usually fixed and known so that measurement of the time etween compressional wave generation and detection is sufficient to determine the compressional wave velocity of the formation. For better accuracy, such distance is usually much greater than the dimensions of the source or detector. Information important for production of oil and gas from subterranean earth formations may be derived from the compressional wave velocities of such formations.
When a compressional wave generated by a compressional wave source in the borehole liquid reaches the borehole wall, it produces a refracted compressional wave in the surrounding earth formation as described above. In addition, it also produces a refracted shear wave in the surrounding earth formation, and guided waves which travel in the borehole liquid and the part of the formation adjacent to the borehole. Part of such shear wave is refracted back into the borehole liquid in the form of a compressional wave and reaches the detector in the logging sonde. The guided waves are also detected by such detector. Any wave that is one of the three types of waves detected by the detector may be called an arrival: the compressional waves in the borehole liquid caused by refraction of compressional waves in the formation the compressional wave arrivals, those caused by refraction of shear waves in the formation the shear wave arrivals, and those caused by guided waves the guided wave arrivals. Thus, the signal detected by the detector is a composite signal which includes the compressional wave arrival, and the shear wave arrival and the guided wave arrivals. Compressional waves travel faster than shear waves and shear waves usually travel faster than the guided waves. Therefore, in the composite signal detected by the detector, the compressional wave arrival is the first arrival, the shear wave arrival the second arrival, and the guided wave arrivals the last arrivals. In measuring the compressional wave velocity of the formation, the time interval between generation of compressional waves and detection of the first arrival detected by the detector gives the approximate travel time of the refracted compressional wave in the formation. Hence the later shear wave and guided wave arrivals do not affect measurement of compressional wave velocity of the formation.
In addition to traveling over a vertical distance in the formation approximately equal to the distance between the source and detector, the compressional wave also travels over short distances in the liquid. The extra time required to travel such short distances introduces errors in the velocity log. To reduce such errors, conventional logging devices employ at least two detectors spaced vertically apart along the borehole from each other. The timer interval between detection by the two detectors is measured instead of the time interval between transmission and detection. The ratio between the distance between the two detectors and such time interval yields the compressional wave velocity. Since the compressional wave travels over approximately equal short distances in the borehole liquid before reaching the two detectors, the time interval between detection by the two detectors is a more accurate measure of the actual level time in the formation. Therefore, using two detectors and measuring the time between detection by the two detectors yield a more accurate compressional wave velocity. Other spurious effects such as borehole-size changes and sonde tilt may be reduced by conventional devices. One such device is described in Log Interpretation, Volume 1 - Principles, Schlumberger Limited, New York, N.Y. 10017, 1972 Edition, pages 37-38.
It is well known that shear wave velocity logging may also yield information important for production of oil and gas from subterranean earth formations. The ratio between the shear wave velocity and compressional wave velocity may reveal the rock lithology of the subterranean earth formations. The shear wave velocity log may also enable seismic shear wave time sections to be converted into depth sections. The shear wave log is useful in determining other important characteristics of earth formations such as shear stress, porosity, fluid saturation and the presence of fractures. The shear wave log may also be helpful for determining the stress state around the borehole which is very important in designing hydraulic fracture treatments.
The conventional compressional wave logging source and the compressional waves it generates in the borehole liquid are symmetrical about the logging sonde axis. When such compressional waves are refracted into the surrounding earth formation, the relative amplitudes of the refracted shear and compressional waves are such that it is difficult to distinguish the later shear wave arrival from the earlier compressional wave arrival and from the reverberations in the borehole caused by refraction of the compressional wave in the formation. Therefore it is difficult to use a conventional symmetrical compressional wave source for logging shear wave velocity. Correlation techniques have been employed to extract the shear wave arrival from the full acoustic wave train recorded. Such techniques, however, usually require processing of data by a computer so that shear wave velocities cannot be logged on line. It may also be difficult to extract the shear wave arrival if it is close in time to the compressional wave arrival.
Asymmetric compressional wave sources have been developed for logging shear wave velocity. Using such sources, the amplitude of the shear wave arrival may be sigificantly higher than that of the compressional wave arrival. By adjusting the triggering level of the detecting and recording systems to discriminate against the compressional wave arrival, the shear arrival is detected as the first arrival. It may thus be possible to determine the travel time of shear waves in the formation and therefore the shear wave velocity. Such asymmetric sources each generates in the borehole liquid a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. The interference of the compressional waves may cause the shear wave arrival to be stronger than the compressional wave arrival. Asymmetric sources are disclosed by Angona et al. European Patent Application No. 31989, White, U.S. Pat. No. 3,593,255, and Kitsunezaki, U.S. Pat. No. 4,207,961.
Angona et al disclose a bender-type source which comprises two circular piezoelectric plates bonded together and attached to a logging sonde. When voltage is applied across the two piezoelectric plates, the plates will bend. The bending of the transducer plates creates a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. White discloses a compressional wave source comprising two piezoelectric segments, each in the shape of a half hollow cylinder. The two segments are assembled to form a split cylinder. The two segments have opposite polarization and electric voltage is applied to each segment causing one segment to expand radially and simultaneously causing the other segment to contract radially thereby producing a positive compressional wave in one direction and simultaneous negative compressional wave in the opposite direction. In Kitsunezaki, coils mounted on a bobbin assembly are placed in the magnetic field of a permanent magnet and current is passed through the coils to drive the bobbin assembly. The movement of the bobbin assembly ejects a volume of water in one direction and simultaneously sucks an equivalent volume of water in the opposite direction, thereby generating a positive pressure change in one direction and a simultaneous negative pressure change in the opposite direction.